Fireplaces have been a part of permanent dwellings since such dwellings were first built. In the early years before central heating was developed, fireplaces were an important source of the heat that warmed these dwellings and their occupants. However, after central heating became available, the greater convenience and efficiency of central heating relegated fireplaces to an esthetic function for the most part.
One long-standing problem with fireplaces is the inconvenience and mess of burning wood. It is relatively difficult to start a wood fire. Once that has been done, it is necessary to continuously add further wood to maintain the fire. It is not easy to shut down a wood fire. Instead the occupant must allow it to burn itself out, during which time cold air can flow down the flue, cooling the room air. Then, after waiting for the ashes to completely cool which may take a day or more, the occupant must remove and discard the ashes. This last is a dirty and tedious job. Ashes are dusty, and the fine particles drift throughout the room during ashes removal.
For these reasons, gas-fueled fireplaces are becoming more and more popular. They are easy to start and stop, and they produce little or no soot and essentially no ash. An artificial log or two provide a wood fireplace ambiance, and a hidden burner directs a flow of gas to feed the flame and to form a combustion site within the fireplace.
More recently fireplace appliances or inserts have been developed that substantially improve fireplace efficiency. These appliances include a heat exchanger receiving heat from the combustion site for warming room air. A circulating fan forces room air through the heat exchanger. One significant disadvantage of most of these inserts is that they require line electrical power to operate the circulating fan. Thus, they are inoperable during power outages, when they""re frequently needed most. Secondly, particularly during installation in existing fireplaces, running line power to a fireplace is expensive.
Recent developments have addressed this problem to some extent. For example, U.S. Pat. No. 6,037,536 (Fraas) shows a fireplace insert using a panel of photovoltaic devices to convert infrared radiation energy to electrical energy. This design has the potential to provide a substantial amount of power, and more than enough to operate a circulating fan. However, the overall design may not be well suited for heating room air. And the photovoltaic devices may be expensive and require frequent cleaning for good efficiency.
Accordingly, there are good reasons to seek a different technical approach when the aim is improve the ability of a fireplace to heat a room. Thermoelectric devices such as thermopiles have been available for many years, used for example for sensing presence of pilot flame in a burner. The pilot flame produced sufficient heat to produce a current allowing a solenoid to hold a gas valve open. However, until recently, thermopiles produced power measured in the hundreds of milliwatts at most, which is much less than needed to operate a fan for drawing air from a room for heating using fireplace combustion. Further, these thermopiles had cylindrical shapes not well suited for the aesthetics of a fireplace.
Recently more efficient thermoelectric devices have been developed that are formed as a plate or layer, hereafter referred to as a thermoelectric layer. The thermoelectric layer has a heat-receiving surface facing in a first direction and a heat-rejecting surface facing generally in a direction opposite to the heat-receiving surface. One such device designated as the HZ-2 thermoelectric module is currently available from Hi-Z Technology, Inc., 7606 Miramar Rd., San Diego, Calif. 92126-4210. The HZ-2 device has a bismuth-tellurium semiconductor layer (hereafter Bixe2x80x94Te layer) and is about 1.15 in. (2.9 cm.) square and 0.2 in. (0.5 cm.) thick. The HZ-2 device provides over 2 watts of electrical power when its heat-receiving and heat-rejecting surfaces are held at a 200 C. temperature difference. A number of HZ-2 modules can be combined to provide more power. Further discussions of this technology are found in U.S. Pat. Nos. 5,769,943; 5,610,366; and 5,747,728.
We have developed an appliance for efficiently heating room air from the heat of a flame having a combustion site within a fireplace. The appliance is to be placed within the fireplace cavity.
The appliance includes an airflow path having an inlet duct for receiving room air and an outlet duct through which this air returns to the room, and has a heat exchange duct between the inlet and outlet ducts. The inlet, heat exchange, and outlet ducts collectively define or form the airflow path.
A fan is mounted within the airflow path to force room air through the airflow path from the inlet duct to the outlet duct and through the heat exchange duct. A motor is mechanically connected to operate the fan.
A thermoelectric generator is mounted to receive heat from the flame and to provide electrical power at an electrical terminal. A heat sink is mounted in the heat exchange duct and in heat exchanging relationship with the thermoelectric generator.
Air flowing through the heat exchange duct is heated by the heat sink. The airflow removes heat from the heat sink, thereby holding the heat sink cool relative to the temperature of the thermoelectric generator where the heat from the flame is received from the combustion site.
One version of this invention includes an electrical connection between the thermoelectric generator""s electrical terminal and the motor. The motor receives electrical power from the thermoelectric generator and operates the fan. The fan causes airflow through the heat exchange duct, which cools the heat sink by heating the air. The heated air flows back into the room, thereby warming the room.
A preferred version of the invention includes a thermoelectric generator having thermoelectric material with a heat-receiving surface for mounting adjacent to the combustion site and a heat-rejecting surface in heat-transferring relation with the heat sink.
The thermoelectric generator may include a heat-receiving plate having a first surface to be mounted facing the combustion site, and a second surface oppositely facing from the first surface and in heat-transferring contact with the thermoelectric material""s heat-receiving surface. The heat sink is in heat-transferring contact with the heat-rejecting surface of the thermoelectric material.
One problem that a commercial embodiment must address is the startup dynamics. After the flame first occurs, there will be little heat gradient between the heat-receiving and heat-rejecting surfaces of the thermoelectric generator. Accordingly, little power will be generated. If the heat-rejecting surface temperature rises quickly as the heat-receiving surface warms, the thermoelectric generator will produce little or no power. In this case, the fan may fail to operate, with the result that no cooling airflow across the heat sink occurs. The situation may lead to temperature runaway for the heat sink, with the fan failing to ever operate.
We have developed a number of solutions to this problem. One of these solutions comprises using a heat sink having a large thermal mass. As the heat is applied to the thermoelectric generator""s heat-receiving surface, the large thermal mass of the heat sink keeps the heat-rejecting surface of the thermoelectric generator sufficiently cool to allow the fan to begin operating. After the fan begins to operate, the airflow will function to maintain the heat-rejecting surface at a sufficiently low temperature.
A load-reducing feature in the fan may be combined with the high thermal mass heat sink solution, or may be employed alone. Such a feature can in one embodiment comprise feathering or folding fan blades that provide limited airflow while feathered. Such blades require little torque to rotate. As the motor speed builds, centrifugal force causes the fan blades to deploy in an extended position which forces increased airflow through the ducts.
An alternative load-reducing feature may be a clutch for connecting the fan to the motor. Still another type of load-reducing feature may be a small auxiliary fan suitable only for partially cooling the heat sink but that operates on a relatively small amount of power while power is removed from the large main fan. Once the heat-receiving surface of the thermoelectric generator has heated sufficiently, enough electrical power is available to operate the large fan.